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Effect of Shot Peening on Fatigue Strength of Maraging Steel

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Effect of Shot Peening on Fatigue Strength of Maraging Steel Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 Effect of shot peening on fatigue strength of maraging steel N. ~awa~oishi',T. ~a~ano~ & M. ~ori~arna~ I Faculty of Engineering, Kagoshima University,Kagoshima, Japan 2 Miyakonojyo National College of Technology, Miyakonojyo, Japan Abstract Rotating bendmg fatigue tests were carried out for a shot-peened maraging steel in order to investigate the effects of shot peening on the fatigue strength and the fracture mechanism focusing on the effect of surface roughness. Fatigue strength was markedly improved by shot peening because of hardening and generation of compressive residual stress in the surface layer. The origin of fatigue fracture changed from the specimen surface at high stress levels to an inclusion in the interior of specimen at low stress levels. And at the middle stress levels, both fracture modes were observed. Consequently, the shape of the S-N curve of shot- peened specimen was complex, corresponding to the change of fracture mode. In the region where surface fracture occurs, polishing the specimen surface and double shot peening using superhard fine particles were effective to improve the fatigue strength through the decrease in stress concentration due to smoothening the specimen surface. Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 100 Surface Twatment V1 1. Introduction Maraging steel is an ultra-high strength steel which has both high tensile strength and high ductility [1],[2]. However, fatigue strength is relatively low in comparison with the hgh static strength [3]. Therefore, the study on the improvement of fatigue strength of maraging steel is important. Shot peening is one of effective methods for improving the fatigue strength [4]. However, the increase in surface roughness due to shot peening is very harmful to fatigue strength, because notch sensitivity of high strength steel is very high. In the present study, rotating bending fatigue tests were carried out for shot- peened maraging steel in order to investigate the effects of shot peening on the fatigue strength and the fracture mechanism focusing on the effect of surface roughness. 2. Material and experimental procedures The material used was an 18% Ni maraging steel whose chemical Table 1. Mechanical properties. - Reversmn 1 I austen'te7vol.% Agw ooc UA 01 1 16 cond~t~on MPa ' MPa I MPa I % / i I 753K-48ks ! 834 0 / 2073 2156 600 / BA Table 2. Shot peening conditions. Blastmy equlprnon! Air type I I Cemented a0 carblde 0 3 Ptessura ( MPa ] 0.3 0.3 0 3 I I Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment V1 10 1 Aging condition for fatigue test I 1 10 102 103 Aging time , ks Figure 1:Agmg curve. Figure 2: Shape and dimensions of specimen. composition in weight was O.O05C, O.O5Si, O.O3Mn, 18.7Ni, 5.01M0, 8.94C0, 0.9.2Tq 0.12A1 and remainder Fe. The material was solution treated for 5.4 ks at 1123K in vacuum, followed by air cooling and age hardening in a salt bath. Figure 1shows the aging curve and the condition used for the fatigue test. As the aging condition for the fatigue test, the underaging condition of 753K-48ks was selected considering that the material had sufficiently @h hardness and high fatigue strength, and that a marked effect of shot peering was confumed in the prelirmnary test. Table 1shows the mechanical properties. Figure 2 shows the shape and dimensions of specimen. ARer machining, all the specimens were electro-polished by about 20 p m hmthe surface layer. Table 2 shows the shot peening conditions. Distributions of hardness and residual stress were measured by using a micro Vickers hardness tester and an X ray diftjraction device, respectively Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 102 Surface Deatment V1 Surface roughness was measured using a stylus-type profilometer and was the average value of five points of maximum roughness Ry. Fatigue tests were carried out using an Ono-type rotating bending fatigue testing machine with a capacity of 15 N m operating at about 50 Hz. Shot peened surface was observed using a scanning electron microscope. 3. Experimental results and discussion Figure 3 is examples of SEM photographs of shot peened surfaces. Surface roughness was markedly improved by double shot peening. Figures 4 and 5 show the distributions of hardness and residual stress, respectively. By shot peening, the surface layer was hardened and Ry12 ,U m Figure 3: Shot peened surface. 600 0 100 m 3W m0 500 Depth from the surface , ,xrn Figure 4: Hardness distribution. Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment V1 103 compressive residual stress was generated. The hardening was hgh in case of double shot peening, though the distribution of residual stress did not change, except for the one near the surface. S-N curves of electro-polished specimens and specimens peened by 1.1 mm shot are shown in Q. 6, in which solid marks and open marks indicate an internal fracture and a surface fracture, respectively, and half solid marks indicate a combined fracture of internal and surface as will be described in detail hereinafter. Fame strength is strongly increased by shot peening and the shape of S-N curve of peened specimen is complex. l%gure 7 is an example of a crack initiated at peened surface at high stress level. Cracks initiated at the stress concentrated parts like a rugged or 0 1W 2a, XI0 400 500 Depth from the surface , urn Figure 5: Distribution of residual stress. Number of cycles N Figure 6: S-N curves. Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 104 Surface Deatment V1 Figure 7: Initiated crack at' shot peened surface ( 4 1.lmm). @)oO= I 100m~~,X=%.7x10~cycfes (ch=750A%Ba, ~=5.4x10~cycles Figure 8: SEM photographs at several stress levels. sunken part of peened surface. Figure 8 is SEM photographs showing the fracture surfaces at several stress levels. Fracture occurred hmspecimen surface at high stress levels, while an internal fracture occurred at low stress levels. And at the middle stress levels, both hcture modes were observed. In general, hcture made is either the surface or the internal of specimen, consequently S-Ncurve showed a double- step wise in surface hardened steels [51,[61. In this case, however, combined hcture existed at middle stress levels. This may be the reason why S-N curve does not show dehite step. Moreover, ktigue strength for the surface fracture may be increased by improvement of surface roughness and Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment V1 105 distributions of hardness and residual stress. That is, it is effective to smmthen the surface and to maximize the peek of hardness and compressive residual stress near the surface. Figure 9 shows S-N curves for specimens electropolished the peened surface by about 100,~m. In the figure, the results of double shot peened specimens using super hard heparticles are also shown. Fatigue strengths are markedly improved, in spite of the ~ decrease in the depth of hardened layer due to electropolishmg. Moreover, the similar effect is obtained by double shot peening. However, the influence of shot peening condition on the fatigue strength in long life region, where the internal fracture occurred, is very small. In the following, the improvement of fatigue strength in long life region will be investigated. Figure 10 is SEM photographs magnified the crack initiation site and the boundary between the internal crack and the surface crack. A crack initiates hman inclusion and the surface is relatively flat around the inclusion, though a rough granular area is observed near the inclusion. Moreover, many intergranular cracks are observed at the boundary between the internal crack and the surface crack. Murakami et al. [71 investigated the role of hydrogen brittleness in the early growth of fish eye cracks and indicated that a rough surface area can be observed in the vicinity of inclusions which caused internal hctures after a very hqgh fatigue cycles. The formation of rough surface around inclusions was recounted as a result of hydrogen brittleness assisted fatigue crack growth so that crack propagation in this area was extremely slow and discontinuous. On the other hand, authors indicated that the formation of reversion austenite was very 1600 - . .-....1 . ...-.., . .-.....v . ....-.n d DouMe '31 [email protected] P = 1m- 6 Ol.lmm-, €P. 1m- -3,e a 1000- E, Single '31.1 D 800- e " """I . ' . """ m0104 " 1 @ 106 I 07 108 Number of cydes N Figure 9: S-N curves of specimens eledro-polishedpeened surface. Transactions on Engineering Sciences vol 39, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533 106 Surface Deatment V1 (a) Origin of crack initiation (b) Boundary between internal crack and surface crack Figure 10: SEM photographs of internal crack (without reversion austenite). Shot Size dO.6mm . Number of cycle N Figure 11: S-Ncurves of steels with and without reversion austenite. effective to improve the fatigue strength of maraging steel, especially at low stress levels [B]. Moreover, reversion austenite is insensitive to hydrogen brittleness. These results suggest that the fatigue strength in long life region can be improved by formation of reversion austenite. Figure 11 is S-N curves showing the iduence ofthe reversion austenite. The results were obtained by using shot size of O.6mm. In this case, the steel with the version austenite was prepared by aging the material at 783°C for 48 ks to obtain the similar static strength (o~=2043MPa, Hv=627) of the steel without reversion austenite ( o ~=2065MPa, Hv=635).
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